Mocking & Dependency Isolation
Testing in isolation ensures that your unit tests are fast, deterministic, and fail only when the logic inside the unit is broken.
Test Double Types
- Dummy: Passed around but never actually used (e.g., to fill a parameter list).
- Stub: Provides canned answers to calls made during the test.
- Spy: A stub that also records some information based on how it was called.
- Mock: Objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
- Fake: Have working implementations, but usually take some shortcut (e.g., an in-memory database).
Best Practices
- Don't Mock Everything: If a class is a pure data object or a simple utility, use the real thing.
- Mock Only What You Own: Avoid mocking third-party libraries; wrap them in an interface instead.
- Verify Behavior, Not Implementation: Focus on what the mock should receive, not the internal state of the mock.
Example (Python)
from unittest.mock import MagicMock
# Create a mock for an external API service
service = MagicMock()
service.get_user_data.return_value = {"id": 1, "name": "Jane Doe"}
# Use it in your test
result = my_function(service)
assert result == "Jane Doe"
service.get_user_data.assert_called_once_with(1)
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